1. Field of the Invention
The present invention relates generally to a radio frequency modulator that is provided in a set-top box for receiving satellite broadcast signals or in a cable broadcasting receiver and, more particularly, to a radio frequency modulator, which can not only overcome the problem of the degradation of image quality by compensating for a chrominance-to-luminance delay, but also enable different channels to be recorded and watched at the same time and a stereo broadcast to be listened to.
2. Description of the Related Art
Generally, a Radio Frequency (RF) modulator is a device that is provided in a satellite broadcasting receiver (set-top box) or cable broadcasting receiver based on broadcasting methods different from a sky wave broadcasting method. The RF modulator functions to convert received satellite or cable broadcast signals into the channel signals of sky wave broadcasting so that a satellite or cable broadcast can be received by a television.
FIG. 1 is a conceptual view of a typical RF modulator 13. The RF modulator 13 is implemented using a single independent semiconductor package. The RF modulator 13 functions to receive a video signal (VIDEO in) and an audio signal (AUDIO in) that are received by a corresponding receiver and converted into base-band signals, to add the signals, to receive power (MOD B+) and control signals (SCL and SDA), and to output an RF signal in a preset sky wave broadcasting channel band (e.g., channel 3 or 4 band). The channel band of the RF output signal is set by each individual country. The RF modulator 13 is connected to a first switching means 11 that is placed between an RF input terminal (RF input) and an RF output terminal (RF output) and is switched on/off by the control signal (SDA), and a second switching means 12 that is placed between the output terminal of the RF modulator 13 and the RF output terminal (RF output) and is switched on/off by the control signal (SDA).
In the above-described structure, when the operating power (MOD B+) is on, one of an RF signal either received by an antenna and an RF modulated signal obtained by the RF modulator 13 can be selected and output using the control signal (SDA) In contrast, when the operating power (MOD B+) of the RF modulator 13 is off, the first switching means 11 is turned on, and, therefore, the RF signal received by an antenna is output to the output terminal (RF output).
The RF modulator 13 is implemented using an oscillation circuit, a frequency conversion circuit and a filtering circuit, and modulates the base-band video and audio signals to a preset RF-band carrier. Accordingly, the conventional RF modulator 13 outputs only single, fixed channel-band signals.
Meanwhile, as viewers' desire to watch high-quality images and listen to high-quality sound is increasing, plenty of research into the improvement of image and sound quality is being carried out.
In the meantime, the number of homes having two or more televisions is increasing, in which case different viewers may watch different televisions, so that broadcasts that the viewers desire to watch may be different from each other. Accordingly, there is a tendency that a set-top box for receiving a cable or satellite broadcast is developed to transmit video signals of different channels at the same time.
Therefore, there has been the necessity for an RF modulator capable of supporting the above-described viewers' demands.